1. Technical Field
The invention relates to semi-trailer subframes, and in particular to movable subframes for semi-trailers. More particularly, the invention is directed to a movable subframe for semi-trailers, which includes one or more generally X-shaped or cross-shaped structures that replace one or more of the transversely extending parallel cross members of conventional subframes, or alternatively replace the heretofore state-of-the-art transversely extending K-shaped structures, for more securely supporting one or more axle/suspension systems suspended from the subframe and to more efficiently react loads imposed on the subframe during operation of the vehicle, while reducing the overall weight and possibly the cost of the subframe.
2. Background Art
Movable subframes or secondary frames, typically referred to as sliders in the trucking industry, have been utilized on tractor-trailers or semi-trailers for many years. One or more axle/suspension systems usually are suspended from a single slider structure. The slider in turn is mounted on the underside of the trailer body or primary frame, and is movable longitudinally therealong to increase maneuverability and provide a means of variable load distribution. Once properly positioned, the slider is locked in place on the underside of the trailer, usually by a retractable pin mechanism.
Conventional or prior art slider designs were developed before the advent of air suspension systems for trailers. At that time, spring suspension systems were the suspension of choice for trailers. However, the spring suspension system resulted in a relatively rough ride to the cargo and did not equalize in all situations, thus creating the need for a slider with soft ride characteristics and efficient equalization characteristics. The development of the slider resulted in good variable load distribution for trailers, which enabled trailers to be more versatile.
Moreover, the subsequent development of air suspension systems provided load equalization among multiple axles for semi-trailers, with or without the utilization of sliders, as well as improved ride quality for individual axles. Of course, the combination of a movable slider and an air suspension system provided maximum versatility with respect to variable load distribution and load equalization in a trailer. Unfortunately, prior art sliders equipped with air suspensions added unwanted weight to the trailer, primarily because those sliders were originally built to support spring suspensions and adapting them to incorporate air suspensions required additional bracing and support.
Also, vehicles containing more than one non-steerable axle, including semi-trailers, are subject to lateral or side loads. Lateral loads can act through the slider in opposite directions, and the affect of such twisting torsional loads on the slider can be significant. Moreover, a slider is subjected to strong vertical and longitudinal loads. A durable slider design must effectively react such loads. Conventional prior art slider designs control vertical loads by utilizing rigid, and therefore heavy, main members and cross members. However, such a rigid structure fails to minimize the effect of lateral and longitudinal loads on the slider structure.
The current state-of-the-art slider design in the semi-trailer industry is the “K-frame slider,” which is the subject of U.S. Pat. No. 5,720,489, dated Feb. 24, 1998, to Pierce et al., which is assigned to the Boler Company, the assignee of the instant application. This invention replaced at least four of the cross members of conventional prior art sliders with four shorter and lighter diagonal or inclined members, which when combined with the remaining pair of cross members resulted in a pair of K-shaped structures. These transversely extending K-shaped structures improved the ability of the slider to react lateral, longitudinal and vertical loads, while also reducing weight and cost.
While the K-frame slider was an improvement over conventional prior art sliders solely utilizing a plurality of transversely extending parallel cross members, the present invention solves certain problems associated with the K-frame slider and thus is an improvement over both prior art K-frame and conventional sliders. More particularly, the present invention more efficiently reacts the load vectors emanating from side or lateral loads and racking loads by locating cross-braces generally at the locations where those vectors occur. In contrast, the K-frame slider, while performing satisfactorily in most cases, locates its K-shaped structures remote from those particular load vectors, so that certain unwanted deflections and stress risers still occur resulting in less than optimum integrity of the slider secondary frame structure. Thus, the present invention is stronger, reacts loads more efficiently, is less costly to manufacture, and is lighter than prior art conventional and K-frame slider designs.